This invention relates to a space diversity antenna system and, more particularly, to an improvement to such a system which substantially eliminates nulling between multiple antennas.
Effective communications to and from airborne platforms often require multiple antennas. This requirement is imposed both by the beams formed by antennas having gain and the shadowing of the antenna pattern by the airframe as a function of aircraft attitude. When the exact location of a single communication partner is known, it is possible to switch between multiple antennas. However, in a more general case, when the location is not known or when there are multiple partners, simple switching is not effective. In these cases, RF energy must be simultaneously provided to all antennas. This will result in conditions where the energy received, on the airborne platform at both antennas or at the partners"" antennas from both airborne antennas, will create a null. This null results from two paths having equal amplitudes but opposite phases. In these nulls, communication is not possible. It would therefore be desirable to provide an arrangement wherein the effect of these nulls is effectively removed electronically with minimal impact on system hardware and cost and which allows for effective communications to and from airborne platforms utilizing multiple antennas that are simultaneously operated.
Multiple antennas are commonplace on airborne platforms. Elimination of the interference pattern that arise from simultaneous activation has also been a common problem. In the past, interference patterns, or nulling, has been addressed by:
Switching between antennas;
Using full space diversity;
Using multiple frequencies; and
Sending data redundantly (i.e., multiple times).
Each of these approaches has disadvantages. Thus, switching between antennas requires a knowledge of the relative location of the communication partner and precludes multiple partners. Full space diversity requires multiple antennas at all sites, increasing system cost and complexity. Use of multiple frequencies increases system complexity, cost, and may reduce data throughput. Finally, sending data redundantly reduces system data capacity. It would therefore be desirable to provide a system which does not suffer from any of the foregoing disadvantages by requiring a minimum of additional hardware and which is applicable to all wireless communication systems.
According to the present invention, a space diversity antenna system operating at a predetermined block rate comprises a first antenna and a second antenna spaced from the first antenna. The system also includes a source of signals to be radiated from the first and second antennas and circuitry using signals received by the first and second antennas. A transceiver is coupled to the source, the circuitry, the first antenna and the second antenna. The transceiver is adapted to split and route signals from the source to the first and second antennas and to combine and route signals from the first and second antennas to the circuitry. Dither circuitry is interposed in the signal path between the transceiver and one of the first and second antennas. The dither circuitry is arranged to alternately insert and remove a circuit element in the signal path at a submultiple of the block rate. The circuit element is selected from the group consisting of an amplitude attenuator and a phase changer.
In accordance with an aspect of this invention, the system is mounted to an aircraft having a major longitudinal axis and further comprises an inertial sensor providing signals indicative of aircraft attitude about the axis, and an angular positioner including a motor. The positioner is coupled to one of the first and second antennas and is adapted to rotate that one antenna about the axis. A motor controller is coupled between the inertial sensor and the positioner motor and is arranged to receive sensor signals and control the motor to maintain the one antenna at a substantially fixed attitude in inertial space.
In accordance with another aspect of this invention, the dither circuitry comprises a plurality of circuit elements each of the same type and of a different value, and a plurality of pairs of PIN diodes. Each pair of PIN diodes flanks a respective circuit element with the anodes of each pair of PIN diodes being each coupled to a respective end of a respective circuit element.